The present invention relates generally to industrial process transmitters for use in industrial process control systems. More particularly, the present invention relates to diagnostic systems for verifying performance of process transmitters having a gauge pressure sensor.
Process instruments are used to monitor process parameters, such as pressure, temperature, flow and level, of process fluids used in industrial processes. For example, process transmitters are typically employed in industrial manufacturing facilities at multiple locations to monitor a variety of process parameters along various production lines. Process transmitters include sensors that produce an electrical output in response to physical changes in the process parameter. For example, pressure transmitters include pressure transducers that produce an electrical output as a function of the pressure of a process fluid, such as in water lines, chemical tanks or the like. Each process transmitter also includes transmitter electronics and circuitry for receiving and processing the electrical output of the sensor so that the transmitter and process parameter can be monitored locally or remotely. Locally monitored transmitters include displays, such as LCD screens, that show the electrical output at the site of the process transmitter. Remotely monitored transmitters include electronics and circuitry that transmit the electrical output over a control loop or network to a central monitoring location such as a control room. Configured as such, the process parameter can be regulated from the control room by including automated switches, valves, pumps and other similar components in the process control system and the control loop.
It is frequently desirable to perform checks or diagnostics of the process control loop to verify operation and performance of each transmitter within the control loop. More particularly, it is desirable to verify performance of each transmitter remotely from the control room without performing invasive procedures on the control loop or physically removing the transmitter from the control loop and industrial process control system. Currently, diagnostic capabilities are limited to obtaining information relating only to performance of the control loop and transmitter electronics. For example, the control room is able to initiate a test signal that originates from the transmitter electronics and then propagates throughout the control loop. The control room, knowing the magnitude and quality of the initiated test signal, can then verify that the control loop and transmitter respond properly to the test signal. The control room thus mimics sensor output and checks that the electronics and control loop respond in kind. The control loop, however, is not able to verify functionality of the sensor. For example, the mimicked test signal does not verify if the sensor is undamaged and producing a valid pressure signal.
Sensors respond to a physical change in the process fluid, rather than an electrical input. For example, capacitance-based pressure sensors used in pressure transmitters include a fixed electrode plate and an adjustable electrode plate, which typically comprises a flexible sensor diaphragm. The sensor diaphragm is connected to the process fluid through a simple hydraulic system that communicates the process fluid pressure to the sensor. The hydraulic system comprises a sealed passageway positioned between the sensor diaphragm at a first end, and a flexible isolation diaphragm at a second end to engage the process fluid. The sealed passageway is filled with a precise amount of hydraulic fluid that adjusts the position of the sensor diaphragm as the process fluid influences the isolation diaphragm. As the pressure of the process fluid changes, the position of the sensor diaphragm changes, resulting in a change in capacitance of the pressure sensor. The electrical output of the pressure sensor is related to the capacitance and thus changes proportionally as the process fluid pressure changes. Thus, proper verification of the sensor requires physically moving the sensor diaphragm.
Previous attempts at sensor diagnostics have involved using deadweight testers or hand pumps to deliberately increase the pressure of the process fluid or the fill fluid to check if the sensor responds. These methods require that an operator visit the location of the transmitter and that the process transmitter to be taken offline, thus inhibiting automation of the verification process. Other methods involve providing piezoelectric crystals within the fill fluid that create a transient pressure pulse that influences the sensing diaphragm. Other attempts have involved using accelerometers to detect induced vibrations of the fill fluid to compare with corresponding changes in sensor output. It is, however, difficult to control the pulses generated by the crystals or the induced vibrations. Thus, repeatability of the diagnostic is limited and verification of the pressure sensor is inconsistent. It is also difficult to provide activation energy to piezoelectric crystals within the fill fluid or supply adequate power to the accelerometers, as the fill fluid is sealed within the hydraulic system. Furthermore, such systems add considerable expense to the transmitter and manufacturing thereof. There is, therefore, a need for a simple and cost effective diagnostic system and method that accurately verifies operation of sensors in industrial process transmitters.